1. Field of the Invention
The present invention relates to a functional powder advantageously used for water purification or for solid-liquid separation. This invention particularly relates to a functional powder with which substances to separate are combined to be caught and removed from raw water by use of magnetic separation technology.
2. Background Art
Recently, according as industries have been developed and the population has been increased, it has become desired to use water resources effectively. Accordingly, it has become very important to reuse abandoned water such as industrial wastewater. For the purpose of that, it is necessary to purify water, namely, to separate impurities from water. There are known various methods of separating impurities from water. Examples of the known separation methods include membrane separation, centrifugal separation, active carbon adsorption, and ozone treatment. Further, it is also known that floating substances can be removed by use of flocculants. Those methods can remove not only oils and/or clay dispersed in water but also eco-harmful chemicals such as phosphorus or nitrogen compounds contained in water. Among the above, the membrane separation is one of the most popularly used methods. However, if oils dispersed in water are removed with a membrane, pores of the membrane are often clogged with the oils and hence the working lifetime of the membrane is liable to shorten. The membrane separation is, therefore, often unsuitable for removing oils from water. When water is polluted with oils such as heavy oil, buoyancy of the oils can be exploited to remove them. For example, heavy oil floating on water surface is brought together with oil fences extended in the water and then sucked up to be recovered from the water surface, or otherwise heavy oil-adsorbent of hydrophobic material is spread on the water surface so that the heavy oil can be adsorbed and thereby recovered.
Meanwhile, there is known a water purification apparatus of solid-liquid separation type. The apparatus comprises a filter through which raw water is filtrated to separate and remove impurities such as organic substances or other foreign substances (which are hereinafter simply referred to as “impurities”). In the purification apparatus, the raw water is led to pass through the filter having fine pores. If the impurities have projected areas (or projected diameters) larger than the pores, they are caught by the filter and, as a result, water having passed through the filter is collected to obtain purified water. However, if this purification treatment is repeatedly carried out with the same filter, the caught impurities are gradually accumulated on the inlet side of the filter and accordingly pressure loss increases to lower the amount of filtrated water. In that case, it is necessary to stop the treatment and to pour purified water reversely so as to wash away and remove the accumulated impurities.
In the case where too fine impurities to remove with the filter must be separated, they are made to cohere with flocculants to form aggregations having enough sizes (namely, sizes of a few hundred micrometers) to catch by the filter and then the aggregations are removed with the filter. For example, flocculants such as aluminum sulfate and poly aluminum chloride are added into raw water to generate aluminum ions or the like in the raw water, and then the water is stirred to aggregate the impurities. Since the impurities cohere to become relatively large aggregations, they can be removed with the filter to obtain purified water having high quality. The separated impurities in the form of aggregations are treated as sludge, which is composted or otherwise is directly conveyed to a landfill site or an incineration plant.
However, the above filter-separation method has some problems to solve.
First, since washing water is made to flow reversely to wash away the impurities accumulated on the filter, the obtained sludge is a mixture of the impurities and the washing water. Accordingly, the sludge produced in the method generally contains a large amount of water. On the other hand, however, the sludge preferably contains water in an amount as small as possible to reduce the conveying cost whether it is composted or directly trucked to a landfill site or an incineration plant. The sludge is, therefore, generally drained with a drying or wringing device such as a spin-dryer or a belt-pressing machine. If the sludge contains water in a large amount, a device capable of draining a large amount of water is needed and hence it costs a lot to buy and run the device.
Further, when the above separation method is performed successively, the filtration process (in which the impurities are gradually accumulated on the filter) and the cleaning process (in which the impurities accumulated on the filter are washed away) must be alternatively repeated. This means that the filtration process must be periodically interrupted to lower the amount of treated water.
Furthermore, in order to treat a large amount of raw water, a large filter is required and hence the purification apparatus must be inevitably enlarged. In addition, from the viewpoint of cost, it is disadvantageous to use flocculants.
As described above, there is room for improvement in the filter-separation method.
JP-A 2000-176306 (KOKAI) discloses a method in which heavy oil is recovered by means of a magnetic separation apparatus. The disclosed method employs magnetic particles which are coated with hydrophobic layers and thereby which are made capable of adsorbing oils. In the method, first those oil-adsorbent particles are spread on raw water to adsorb impurities, namely, to catch heavy oil floating on water. The particles having adsorbed the heavy oil is then pumped up together with the water, and collected by means of a magnetic separation-purification apparatus to recover the heavy oil. Here, the “magnetic separation-purification apparatus” means a device with which the magnetic particles are collected and recovered by use of magnetic force.
The magnetic separation-purification apparatus thus separates and recovers the magnetic particles by use of magnetic force. In addition to the above process, the apparatus can be also used for another purification process. In the process, magnetic particles not coated with hydrophobic layers are added into raw water together with flocculants, so that the magnetic particles serve as nuclei and thereby non-magnetic substances contained in the water are made to cohere around the magnetic particles to form aggregations, which are then separated and recovered with the magnetic separation apparatus by use of magnetic force. In this way, even the magnetic particles not coated with hydrophobic layers can be separated and recovered by the magnetic separation process if only they are pretreated.
However, the present inventors have studied and found that there is still room for improvement. It is found that the magnetic particles coated with hydrophobic layers disclosed in the JP-A 2000-176306 (KOKAI) are insufficiently dispersed in raw water since they have hydrophobic surfaces. The insufficiently dispersed particles cannot adsorb the impurities sufficiently, and hence the impurities are liable to be removed insufficiently.